Our Lapped CPU Heatsink Test Platform

Anyone who's been reading SPCR for even a short portion of our 10+ year history
knows that we take consistency and repeatability seriously. Our vision for SPCR
is a database library for computer gear acoustics and thermal behaviour consistent
enough so that you can compare something reviewed today with one in the same
class of gear reviewed years ago. This usually means using the exact same base
components in a test platform, using the same high precision measurement tools
(whether it be for degrees, decibels, watts, grams, millimeters or millivolts).
We've worked hard to stay true to this vision.

For testing CPU heatsinks, we've maintained from the start that a real CPU
and a real motherboard are are necessary, they cannot be replaced with any simulations,
no matter how much more convenient they may be. Not only does a CPU covered
with an Integrated Heatspreader (IHS) simply not behave the same way thermally
as a CPU-shaped copper plate heated by an electric coil beneath it, the heatsink
mounting hardware can rarely be attached the same way as on a motherboard. The
mounting system and its ease of use, its ability to apply high, even pressure
to the CPU/heatsink interface are critical. This aspect of heatsink testing
usually gets shortchanged when a non-CPU/motherboard test platform is used.

THE MOVE TO A 130W TDP i7-965 EXTREME TEST PLATFORM

But as CPUs, sockets and motherboards came and went through their increasingly
shorter life cycles, we've had to upgrade our test platform periodically. This
means there are a few unavoidable discontinuities in our database. Besides,
there's little point comparing, for example, a heatsink for a 65W P4 on a 775
socket against a heatsink for a 130W i7 on a LGA2011 socket: They are completely
incompatible.

The D950 was not challenging enough any more: The D950 CPU simply
didn't run as hot as the latest socket 1366 i7 processors that had become
the new performance kings at the time. The old platform could not differentiate
enough between the high performance heatsink models designed to take on processors
that run 35W hotter. A hotter 130W 1366-socket processor would help differentiate
the A+ rank from the A and A- performers.

No more replacement parts, mainly, the motherboard of the old platform:
A motherboard used as a CPU heatsink testing platform takes a huge amount
of abuse. Intel informed me years ago that CPU sockets are designed for maybe
20 reliable uses. Oh, it will generally survive beyond that number of CPU
insertions and removals, but the point is that it's reasonable for motherboard
and motherboard component makers to build them for a moderate amount of handling.
CPUs, motherboards and heatsinks are components that usually get seriously
handled only a few times in their lifespan, during installation, a CPU upgrade,
or maybe a heatsink upgrade. But our CPU heatsink test platforms get handled
 and manhandled  every time we test a heatsink. The CPU gets pulled
off and put back on at least a couple times, and the heatsink is often installed
and removed as many as half a dozen times in our examination of the installation
process. In fact, the Asus P5Q-EM that was our test platform in 2010 was actually
the second sample of that model used with the D950 CPU for heatsink testing;
we'd already broken the first one. We would not be able to get a third sample;
more unused samples of that model simply could not be found any more. This
was another good reason for a change.

The LGA1366 i7-965E Cooler Test Platform

TROUBLE IN PARADISE?

Since the 3.2 GHz Core i7-965 Extreme CPU and Asus P6T SE motherboard
came into service (nearly four years ago now), we've tested over 40 heatsinks
on this platform. This is not all of the heatsinks we've reviewed, only the
larger coolers designed for top performance. Some 20 or so less ambitious models
were tested on a cooler-running AMD test platform, then later, on a 95W TDP
Intel socket 1155 platform that replaced it.

We  and presumably most of our audience  have been reasonably pleased
with the performance of our 1366 test platform. The newer LGA2011 socket has
not seen a further increase in CPU heat; the TDP of all LGA2011 CPUs remains
at 130W, which keeps the thermal core of our test platform perfectly relevant.
Admittedly, one socket 2011 model, the i7-3970X, is rated for 150W, but this
$1000 item is an outlier; CPU TDP continues trending downwards. Even the heatsink
mounting mechanism for the LGA2011 hasn't really changed much from LGA1366.
Fundamentally, no one had any quibbles with our testing methodology or the accuracy
and relevance of our results. Of course, no one tests CPU coolers at the extremely
low noise levels that we do routinely.

Still, a few heatsink sample suppliers have been less satisfied, and they've
implied once or twice that SPCR could be biased for some brands. One of the
more public of these incidents of supplier dissatisfaction occured a year ago
with the Dark Rock 2 Tower
Heatsink, three samples of which were submitted by be quiet! All
three samples exhibited mediocre performance, especially given the large size
and surface area of the fins. All three had poor contact footprints between
their bases and the CPU, which seemed largely responsible for the poor performance.

The bequiet! representative questioned whether our testing method could be
flawed, because, apparently, other reviewers had achieved good results. He even
asked whether our use of voltage control instead of PWM control for the cooling
fan could have played a part in the poor results. In an effort to dispel any
hint of foul play or error on our part, I took extra steps to examine and re-test
the three samples in great detail, adding an
extensive postscript that conclusively showed the poor contact between
the Dark Rock 2 Tower bases and the top of our test LGA1366 CPU.

That postscript contains two key photos and captions which I reproduce here:

Imprint of Thermal Interface Material on CPU and base of Dark Rock 2.
A line on the top and bottom edges of the heatsink, matched on the CPU,
shows where the TIM has been pressed away. Yet, the TIM in the center
still has those fractal-like striations. This is conclusive evidence that
the base of the bequiet! Dark Pro 2 sample here is concave.

Desired
TIM imprint with the same CPU/motherboard, and a Prolimatech Megahalems
heatsink. The heatsink was turned 90 degrees for the above photo, and
the TIM was smudged a bit during removal, but you can clearly see the
matching imprints of the TIM on the base and on the CPU. Both have a central
area where there is very little TIM; it's been pushed from the center
due to very tight contact. Only around the periphery are there any striations
of the TIM. This cooler performed a whopping 12°C better than the
Dark Rock 2.

These images are reproduced because they show how and why the profile of the
heatsink base can have a dramatic effect on CPU cooling performance. The left
photo shows that the contact between the DP2 base and the CPU surface was actually
tightest along the perimeter of the CPU rather than at the center,
where there was an obvious gap and no metal-to-metal contact. Since the center
is where the CPU die resides, this is where it is hottest, and where the bond
between heatsink and IHS is critical.